Jones Images of Comet Hale-Bopp

Observer: Terry Jay Jones
Location: ???
Date: April 1997

IR Imaging Polarimetry Comet Hale-Bopp

After two observing runs in 1996 with no measurable polarization, Hale-Bopp
finally decided to cooperate in April of 1997. Below is a series of panels
showing the intensity, edge filtered and polarization of Hale-Bopp at K (2.2
microns).

Intensity Image

At right is a normal intensity
image of the comet at K. Note how the
combination of the jet activity and the rotation of the comet creates a clear
'sprinkler hose' pattern on the side of the nucleus facing the Sun (SW). If you
look carefully, you will see three jet arcs, each one rotation of the nucleus
apart from the other. Also note that the jet is not a very high contrast
feature in the intensity image, sticking up only 20-30% above the underlying
continuum of the coma.

Edge Filtered

This is the same image run
through a Sobel edge filter. The filter just
takes the derivative of the image and is brightest where the slope in the
intensity is greatest. The bright arcs to the SW are at the leading edges of
the jets.

Percent Polarization

Here is the percent
polarization image. The brightest spots in the
second jet arc are about 11% at 2.2 microns. The darkest areas to the NW are
about 7%. The polarization for the whole comet and the nucles is about 9.5%. If
all of the grains in the coma were the same everywhere, then the polarization
would be the same everywhere as well. The clear presence of the jet in the
polarization image indicates that the grain population in the jet material is
different is some ways than the general grain population lifting off of the
nucleus. Most likely the grains in the jet are significantly smaller than the
grains in the rest of the coma.

Polarization vs. Phase

Normally, the
polarization characterisitcs of comets as a function of
phase (scattering) angle are similar to asteroids. They show a 'negative'
branch (polarization in the plane of scattering) at small phase angles. In
asteroids, this is interpreted as due to a rough surface. In comets this is
usually attributed to large fluffy aggregates of small grains. At 2.2 microns,
Hale-Bopp shows a polarization that lies somewhat in between the normal
behavior of comets in the visual and pure Rayleigh scattering. This is probably
due either to 1) a smaller grain population size in Hale-Bopp than in most
comets 2) the effects of observing at a much longer wavelength where the fluffy
aggregates are begining to become comparable to the wavelength or 3) a
combination of the two.

AAS Poster Text

I. Phase Curve

1. Hale-Bopp shows no negative branch at the
0.5% level at 2.2um.
2. The comet is more highly polarized in the near
infrared at intermediate phase angles than
comets observed in the visual.

II. Intensity Image

1. The jet is a low contrast feature.
2. The radial intensity profile of the jet remained
unchanged over a period of 3 hours.

III. Polarization Image

1. The spiral jet is clearly more highly polarized
than the surrounding coma.
2. The polarization in the jet has a much higher
contrast to the rest of the coma than the
intensity.
3. The polarization is highest on the leading edge
of the jet, at least at this particular epoch.
4. The percent polarization is not constant in the
jet from one rotation period to the next.
5. The more distant jet arc is more highly
polarized than the inner one.

Conclusions

1. At near infrared wavelengths, the dust grains
are probably intermediate in behavior between
fluffy aggregates and Rayleigh particles. This
should place constraints on the size of the
aggregates.
2. The grains in the jet are significantly smaller
than in the surrounding coma.
3. The grain size distribution in the jet material
varies on the time scale of hours.
4. The grains or grain aggregates may still be
breaking up as far out as 30" from the nucleus.
For you extragalactic types, try
IR Polarimetry of
Galaxies For galactic folk, try Interstellar Polarimetry